The surface components of the host cell which specifically interact with paramxyovirus proteins during the adsorption-penetration phase of the infective process with be identified by a new approach, crosslinking technology coupled with immunoadsorption specificity. A variety of reagents with different site specificities will be used to crosslink viral antigen-host cell receptor complexes. These complexes will be selectively precipitated out of cellular extracts by immunoadsorption techniques employing antisera to the viral antigens. Components of the complexes will be identified by a two-dimensional gel electrophoresis mapping technique. The strains of Newcastle disease and Sendai viruses to be used in this study are known for their ability to induce cell fusion and represent a wide range of virulence. Some of these strains when grown in tissue culture cells produce fully assembled viruses containing the viral antigens in precursor form. The cooperative roles of the viral hemagglutinin and fusion will be examined using both whole viruses and their isolated components. These studies will extend our knowledge of the molecular basis for variation in virulence of paramyxoviruses and of the membrane fusion process in general.